Television broadcast systems throughout the world have migrated from the delivery of analog audio and video signals to modern digital communications systems. For example, in the United States, the Advanced Television Standards Committee (ATSC) has developed a standard called “ATSC Standard: Digital Television Standard A/53” (the A53 standard). The A53 standard defines how data for digital television broadcasts should be encoded and decoded. In addition, the U.S. Federal Communications Commission (FCC) has allocated portions of the electromagnetic spectrum for television broadcasts. The FCC assigns a contiguous 6 MHz channel within the allocated portion to a broadcaster for transmission of terrestrial (i.e., not cable or satellite) digital television broadcasts. Each 6 MHz channel has a channel capacity of approximately 19 Mb/second based on the encoding and modulation format in the A53 standard. Furthermore, the FCC has mandated that transmissions of terrestrial digital television data through the 6 MHz channel must comply with the A53 standard.
Digital broadcast signal transmission standards, such as the A53 standard, define how source data (e.g., digital audio and video data) should be processed and modulated into a signal that is transmitted through the channel. The processing adds redundant information to the source data so that a receiver that receives the signal from the channel may recover the source data, even if the channel adds noise and multi-path interference to the transmitted signal. The redundant information added to the source data reduces the effective data rate at which the source data is transmitted but increases the potential for successful recovery of the source data from the transmitted signal.
The A53 standard development process was focused on high definition television (HDTV) and fixed reception. The system was designed to maximize video bit rate for the large high resolution television screens that were already beginning to enter the market. Transmissions broadcast under the ATSC A/53 standard, or legacy encoding and transmission standard, present difficulties for mobile receivers.
Recognizing this fact, in 2007, the ATSC announced the launch of a process to develop a standard that would enable broadcasters to deliver television content and data to mobile and handheld devices via their digital broadcast signal, commonly known as ATSC M/H. Changes to the legacy transmission standard include an additional encoding scheme to introduce further data redundancy. The additional encoding has been adapted to better perform with advanced receivers in mobile, handheld and pedestrian devices while still remaining backward compatible with the legacy A53 standard. The proposed changes also allow operation of existing ATSC services in the same radio frequency (RF) channel without an adverse impact on existing receiving equipment.
Referring to FIG. 1, an exemplary ATSC broadcast system 10 including a standard (TS Main) system 12, i.e., existing or legacy system, and M/H (mobile/handheld) system 14 is illustrated. Each system 12, 14 feeds a M/H framing or signaling channel 16 which established and controls the specific structure for each system. After the data packets are processed by the M/H framing channel 16, the data packets are passed to a RF transmission system 18 which includes a channel coding unit 20 for encoding the data packets and a modulator 22 for modulating the data packets onto a carrier signal for transmission via antenna 24.
In transmitters that comply with the mobile ATSC standard, referred to as ATSC M/H, it may be necessary to perform a periodic reset of the trellis encoder, disposed in the channel coding unit 20 used with legacy broadcasting, to a known (e.g. all-zero) state. This reset is typically achieved by replacing bits entering the encoder at pre-defined positions in the bit stream with feed-back from the trellis encoder state storage. Furthermore, since the bits entering the trellis encoder first pass through the non-systematic Reed-Solomon (RS) encoder used with legacy broadcasting, this trellis encoder input replacement results in an integrity violation of the corresponding RS codewords (that was initially computed using known placeholders for the actual trellis reset bits inserted downstream). Therefore, those distorted RS codewords must be retroactively re-computed using the actual trellis reset byte values to produce new non-systematic RS parity bytes that would not cause a RS decoder in a legacy ATSC receiver to flag an error. It is necessary to perform the computations for the new RS parity byte values in a manner that permits the completion of the computation prior to final input to the legacy trellis encoder. It is therefore desirable to include an efficient encoding process and apparatus for recomputing RS codewords in conjunction with a trellis state reset condition.